1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly, to an image forming apparatus for preventing stacking failure of discharged recording sheets and alignment failure at the time of post-processing on recording sheets due to toner fusion and for suppressing a reduction in productivity.
2. Description of the Related Art
In a conventional image forming apparatus in which a toner image is thermally fixed to a recording sheet, toner fusion sometimes takes place between stacked recording sheets, which are raised in temperature at thermal fixing, when post-processing is carried out thereon. As a result of the toner fusion, toner images are peeled off from recording sheets and stacking failure of recording sheets is caused, which poses a problem.
To obviate this, it has been proposed to cool a transfer guide member by means of a cooling fan disposed near a sheet discharge port, thereby cooling recording sheets before being subjected to post-processing (see, for example, Japanese Laid-open Patent Publication No. 2006-349755).
Moreover, for a case where recording sheets such as OHP sheets between which toner fusion easily occurs are used, there has been proposed a cooling system in which the discharge of recording sheets onto a stacking tray is temporarily delayed, thereby cooling the recording sheets (see, for example, Japanese Laid-open Patent Publication No. 2003-248349). With this cooling system, however, when applied to an image forming apparatus having a fear that toner fusion occurs even between ordinary sheets, the sheet discharging time interval must be increased at the time of post-processing on the ordinary sheets, resulting in a high possibility that user's demand on improved productivity cannot be satisfied.
To solve this problem, it has been proposed to detect the toner density on each recording sheet and change the sheet discharge interval, if the detected density is greater than a critical density at or above which toner fusion takes place (see, for example, in Japanese Laid-open Patent Publication No. 2006-243498).
In a small machine demanded to be compact in size and low in cost, however, conventional cooling means such as a cooling fan for cooling recording sheets cannot positively be adopted. Especially in a small machine for office use, a thermal fixing mechanism is disposed adjacent to a sheet discharging part, and therefore, it is difficult to find an installation space for a cooling fan. Since a sheet discharge tray is small in size, a cooling fan is also difficult to be installed on the sheet discharge tray.
In the conventional arrangement, the sheet discharging time interval for recording sheets between which toner fusion is liable to occur is changed in accordance with the determined toner density, and the sheet discharge interval for the next recording sheet is increased when the toner density on the preceding recording sheet is determined to be greater than the critical density.
The conventional arrangement is therefore effective for a machine in which image formation on each recording sheet is started after the toner density on the preceding recording sheet is determined. Such an arrangement is also effective for a machine (such as an image forming apparatus), though in which the image formation interval is long, but which includes a speed-up mechanism to decrease the sheet discharge interval.
However, in a machine in which a transfer path is short in length and a speed-up mechanism is not included, the sheet discharge interval is short and the next image formation is started before completion of the determination of the toner density on the preceding recording sheet. This makes it difficult to selectively increase the next sheet discharge interval in accordance with the preceding image density.
If the sheet discharge interval is controlled to always be made large, on the other hand, the productivity is lowered and the usability is largely impaired.